ASME Y14.5-2009 is a standard that establishes uniform practices for stating and interpreting dimensioning, tolerancing, and related requirements for use on engineering drawings and in related documents. Section 4 of the standard relates to datum reference planes. Datums are theoretically exact points, axes, lines, and planes. A datum reference frame is formed by three mutually perpendicular intersecting datum planes. Section 4 establishes datum features for establishing relationships imposed by geometric tolerances and for constraining degrees of freedom. Section 4 also sets forth criteria for establishing datums and the datum reference frame.
Companies engaged in design and manufacture of mechanical parts utilize probes and coordinate measuring machines to perform dimensional inspection of mechanical parts. The dimensional inspection of a manufactured part includes sampling features of size on the part to establish a datum reference frame, and using the datum reference frame as the basis of measurements of the manufactured part. The datum reference frame may be used by machinists, toolmakers, and quality control inspectors to ensure that the manufactured part agrees with a product definition of the part.
Section 4 of ASME Y14.5-2009 standard does not address the situation where secondary or tertiary datum features of size are non-orthogonal to the other datum planes or non-orthogonal to other datum features. When sampling holes, for instance, an inspector makes assumptions as to where to sample the holes. These assumptions lead to ambiguity, since different inspectors make different assumptions. If datums are ambiguous, repeatability is compromised. As a result, some parts will pass inspection while identical parts will not.